Although not exclusively, the present invention applies more particularly to the guidance of aircraft, such as in particular civilian transport airplanes, throughout the flight to comply with a target passage time at a given point. This point can in particular be located in a landing phase (descent, approach). It is known that a landing phase is normally monitored and managed by air traffic controllers. One of the tasks of an air traffic controller is to ensure that, at a particular (convergence) point in space, the various aircraft converging towards this particular point comply with a sequence of arrival times. This sequence of arrival times at this point can be supplied to him, for example, by a standard system located on the ground which automates the traffic planning. The air traffic controller must then give guidance instructions to the crews of the various aircraft, to maintain a satisfactory separation between two successive aircraft and to ensure that said aircraft comply with the sequence of arrival times at said convergence point with a certain accuracy.
In areas with high traffic density, such a standard sequencing management represents a significant workload for an air traffic controller, which is made increasingly complex because of the continual growth in air traffic.
Furthermore, the time lapse between two successive instructions from the controller to one and the same crew can become relatively great, of the order of some tens of seconds, and even sometimes be as long as a minute. Such a guidance of the aircraft, based on instructions transmitted from the ground, therefore does not offer sufficient accuracy and must be offset by imposed separation minima between two aircraft that are sufficiently great. This of course has the drawback of limiting the density of the air space.
Furthermore, such a standard sequencing management makes it possible to deliver clearances for paths that are optimized in terms of traffic flow management, but to the detriment in particular of parameters specific to each aircraft, which can be very sensitive, for example for airlines, such as fuel consumption, engine wear, arrival time at the airport or sound emissions. In particular, it is known that, in very dense traffic, the air traffic controller often has a very small deconfliction horizon (of the order of a few minutes) and frequently uses a technique called “path stretching” to accurately sequence the aircraft. This standard technique involves reducing or increasing the length of the path of an aircraft, to adjust the passage times of the various aircraft at a given point. More often than not, such a procedure is performed by radar vectoring, by placing the aircraft on a level at low altitude. This delayed adjustment of the path, associated with a guidance that is not generated by a flight management system of the aircraft, does not make it possible to deliver an optimized path for said aircraft, and it often proves costly in particular in terms of fuel consumption and environmental impact (sound emissions, gas emissions, etc.).
Document FR-2 888 636 discloses a device to assist in the approach, with a view to a landing. This device is mounted on an aircraft and uses a standard flight management system and guidance system.
Moreover, a method is known according to which:    a) an operator enters at least one time constraint of RTA (Required Time of Arrival) type, relating to a required passage time at a particular check-point, in particular a convergence point as mentioned above, in a flight of the aircraft;    b) the values of parameters relating to the flight of the aircraft, such as wind or temperature for example, are determined; and    c) said values and said time constraint are used to determine guidance instructions that make it possible to guide the aircraft so that it passes said particular check-point at said required passage time; and    d) these guidance instructions are applied to the aircraft.
The entry of an RTA time constraint relating to a particular check-point, for example in a Flight Management System (FMS), makes it possible to fly the aircraft along a path (called reference path) which is optimized in terms of aircraft parameters, but it does not provide the controller with a simple solution for safely accomplishing the sequencing of the aircraft at the desired check-point.
In practice, in particular, the flight management system does not anticipate the fact that the parameters (for example wind, temperature, the position of the aircraft) taken into account in the calculation of the guidance instructions can be unreliable. These parameters which are entered into the flight management system (including values entered at the waypoints and interpolations made between the waypoints) can be significantly different from the actual parameters that will be encountered by the aircraft from its current position to the point where the time constraint is entered. In particular, with respect to wind and temperature, the difference between the entered values and the actual values can be fairly great, mainly because of the poor accuracy of the weather data concerning wind and temperature, which are transmitted to the crew. This difference therefore renders the abovementioned standard method unreliable. In particular, this standard method does not offer a guarantee of compliance with an RTA time constraint entered at a given point, and this even if the flight management system initially announces (on completion of the entry of this time constraint) that the latter will be complied with. These limitations of the RTA function therefore render its operational use difficult.
Document U.S. Pat. No. 5,121,325 discloses a system for calculating and displaying a range of possible arrival times of an aircraft at a particular point. The minimum arrival time of this range corresponds to the case where the aircraft flies with the highest cost index, and the maximum arrival time corresponds to the case where the aircraft flies with the lowest cost index. This range of arrival times therefore concerns estimates of arrival times at a check-point concerned for speeds of the aircraft ranging from the lowest to the highest. However, these estimates are calculated only with the winds that have been entered into the flight management system, without taking account of possible errors regarding the wind values used. The range (or RTA window) displayed is therefore unreliable, since no guarantee is given to the pilot that the times displayed can be complied with, in particular in the case where the entered winds (or other entered parameters) are wrong, which often happens.